add a VersionMap type

[dhellmann]

Downstream we have a few places where we define a dictionary mapping versions to build settings (source URLs, flags, etc.). We've repeated the pattern enough that it makes sense to add it to fromager as a utility class.

The class should act like (subclass?) a dict. The keys should be Version objects. The values should be arbitrary types.

The instance should have a method named something like resolve() that takes a Requirement and constraint and returns the value associated with the version that matches. It should raise an appropriate exception if no version in the map meets the requirement.